Efficient ternary organic solar cells based on immiscible blends (original) (raw)
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Scientific Reports
Sin, chaneui park & Kilwon cho A low-bandgap acceptor (itic) was added to a binary system composed of a wide-bandgap polymer (pBt-ott) and an acceptor (pc 71 BM) to increase the light harvesting efficiency of the associated organic solar cells (oScs). A ternary blend oSc with an acceptor ratio of pc 71 BM:itic = 8:2 was found to exhibit a power conversion efficiency of 8.18%, which is 18% higher than that of the binary OSC without ITIC. This improvement is mainly due to the enhanced light absorption and optimized film morphology that result from ITIC addition. Furthermore, an energy level cascade forms in the blend that ensures efficient charge transfer, and bimolecular and trap-assisted recombination is suppressed. thus the use of ternary blend systems provides an effective strategy for the development of efficient single-junction OSCs. Organic solar cells (OSCs) can be lightweight, flexible, transparent, and mass-producible 1-3. Recent studies have reported single-junction OSCs with significantly increased power conversion efficiencies (PCEs) > 10% 4-8. In a general approach to the fabrication of OSCs, the photoactive layer can be prepared by mixing a light-harvesting polymer as a donor and an electron-accepting fullerene derivative as an acceptor. However, such binary OSCs have relatively narrow light absorption windows, which restricts their photocurrent generation 9,10. In order to increase their light absorption, tandem structures have been introduced. A bottom cell based on a wide-bandgap polymer and a top cell based on a narrow-bandgap polymer are linked in series, which results in complementary absorption of the solar spectrum and boosts the power conversion efficiency of the incorporated cells 11. However, tandem structures have several drawbacks such as their complex fabrication process and high production costs, which limit their practical applications 12. In the past few years, ternary blend OSCs have been developed that exhibit extended light absorption and do not require complicated fabrication processes 13-15. The light absorption spectrum of the third component is generally complementary to that of the light-harvesting polymer and is introduced into the donor/acceptor binary blend 10. The presence of the third component can result in the formation in combination with the other two components of an energy level cascade for charge transfer, and can also enhance the development of the film morphology. Furthermore, ternary single-junction OSCs can be fabricated with a process that is simpler than the complex processes required for the fabrication of tandem OSCs 16-18. Recently, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC) was developed as a narrow-bandgap acceptor. ITIC exhibits strong light absorption in the infrared region and its energy level can be adjusted for compatibility with other absorbing materials, so ITIC-based OSCs have been found to exhibit outstanding performances 19-25. However, ITIC exhibits high photovoltaic performance in combination with only very few polymers because its aggregation properties pose difficulties for the control of the film morphologies of ITIC-based blend films. Moreover, in some cases, ITIC-based OSCs exhibit relatively low fill factors (FFs) because of recombination losses and low electron mobilities 26. For ITIC to act as an efficient acceptor, it is important to control its aggregation 27. Such control can be achieved by mixing ITIC with [6,6]-phenyl-C 71-butyric acid methyl ester (PC 71 BM), which has high miscibility with donor polymers. The resulting mixed acceptor can then act as a light harvester and be miscible with donor polymers without severe aggregation.
Ternary molecules blend organic bulk heterojunction solar cell
The ternary molecules blend organic photovoltaic cell using poly(3 hexylthiophene) (P3HT), poly[[4,8-bis [(2-ethyhexyl)oxy] benzo(1, 2 b: 4, 5 b − − ′)dithiophene-2,6-diyl][3-fluoro-2-[(2 ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated. The performance of the ternary blend was compared with two binary molecules based photoactive layers such as P3HT:PCBM and PTB7:PCBM. It was found that the ternary molecules pho-tovoltaic cell performs better than those with P3HT:PCBM composition. The optical and morphological properties of the ternary blend active layers were investigated, and the results suggest that better photon harvesting medium can be achieved by blending two donor molecules.
Ternary mixing: A simple method to tailor the morphology of organic solar cells
Organic Electronics, 2009
We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron size structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge 2 carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by ~100 mV), which leads to an overall enhancement of power conversion efficiency.
Using Two Compatible Donor Polymers Boosts the Efficiency of Ternary Organic Solar Cells to 17.7%
Chemistry of Materials
The use of ternary organic semiconducting blends is recognized as an effective strategy to boost the performance of polymer solar cells (PSCs) by increasing the photocurrent while minimizing voltage losses. Yet, the scarcity of suitable donors with a deep highest occupied molecular orbital (HOMO) level poses a challenge in extending this strategy to ternary systems based on two polymers. Here, we address this challenge by the synthesis of a new donor polymer (PM7-Si), which is akin to the well-known PM6 but has a deeper HOMO level. PM7-Si is utilized as the third component to enhance the performance of the best-in-class binary system of PM6:BTP-eC9, leading to simultaneous improvements in the efficiency (17.7%), open-circuit voltage (0.864 V), and fill factor (77.6%). These decisively enhanced features are attributed to the efficient carrier transport, improved stacking order, and morphology. Our results highlight the use of two polymer donors as a promising strategy toward high-performance ternary PSCs.
Role of Morphology and Förster Resonance Energy Transfer in Ternary Blend Organic Solar Cells
ACS Applied Energy Materials
Organic solar cells (OSCs) fabricated from ternary blend thin film absorbers are designed to maximize the range of absorption in the solar spectrum and thus increase the short-circuit current density (J SC) of the device. Herein, we report OSCs formed with two different compositions of ternary blend thin films comprising two electron donors and one acceptor, namely, PTB7-Th/ PCDTBT/IT4F and PTB7-Th/PBDB-T/IT4F. We evaluate the role of Forster resonance energy transfer (FRET) and blend morphology to achieve composition-dependent device performance. We observed ≥10% increment in J SC for both the ternary blends as compared to that for the PTB7-Th:IT4F binary blend, resulting in an enhanced power conversion efficiency (PCE) up to 10.34% for the PTB7-Th:PBDB-T:IT4F blend. We provide evidence that the two foremost parameters that control the PCE are blend morphology and FRET between donor components. The improved exciton generation rate for PCDTBT-based ternary blends was achieved, suggesting effective contribution of FRET toward enhanced device photocurrent, whereas the PBDB-T-based ternary blend excelled mainly due to suppressed carrier recombination as a result of favorable orientation with PTB7-Th/ IT4F.
Journal of Material Chemistry A 4, 4252 (2016)
We demonstrate that blending fluorinated molecules in PEDOT:PSS hole transport layers (HTL) induces charge transfers which impact on both charge extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs fabricated with modified HTL and two photoactive polymer blends led systematically to power conversion efficiencies (PCE) increases, with PTB7:PC70BM blend exhibiting PCE of ~ 8.3 %, i.e. ~ 15 % increase compared to pristine HTL devices. A reduced device-to-device characteristics variations was also noticed when fluorinated additives were used to modify the PEDOT:PSS. Shading lights onto the effect of HTL fluorination, we show that the morphology of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated HTL surface energy but that, instead, the OPVs are impacted not only by the HTL electronic properties (work function, dipole layer, open circuit voltage, charge transfer dynamic) but also by alteration of the complex refractive indices (photogeneration, short circuit current density, external quantum efficiencies, electro-optic modelling). Both mechanisms find their origin in fluorination induced charge transfers. This work points towards fluorination as a promising strategy toward combining both external quantum efficiency modulation and power conversion efficiency enhancement in OPVs. Charge transfers could also be used more broadly to tune the optical constants and electric field distribution, as well as to reduce interfacial charge recombinations within OPVs.
Synergistic effect of polymer and small molecules for high-performance ternary organic solar cells
Advanced materials (Deerfield Beach, Fla.), 2015
A ternary blend system with two donors and one acceptor provides an effective route to improve the performance of organic solar cells. A synergistic effect of polymer and small molecules is observed in ternary solar cells, and the power conversion effi ciency (PCE) of the ternary system (8.40%) is higher than those of binary systems based on small molecules (7.48%) or polymers (6.85%).
Binary Organic Photovoltaic Blends: A Simple Rationale for Optimum Compositions
Advanced Materials, 2008
The most promising device structure for organic photovoltaic devices presented to date is the ''bulk-heterojunction'' whereby a hole-conducting (electron-donating) conjugated polymer, such as poly(3-hexylthiophene) (P3HT), is blended with an electron-conducting (electron-accepting) smallmolecular compound, such as a fullerene derivative. The reported strong composition-and thermal-treatment dependence of the power conversion efficiency of such binaries suggests that phase behavior, processing conditions and the resulting microstructure play a dominant role in the performance of devices based on these systems. Here, we propose a simple rationale for selecting the optimum composition of such crystalline/crystalline polymer/small molecule blends. We find that these binary systems feature simple eutectic phase behavior, and that the optimum composition for device performance is slightly hypoeutectic when expressed in terms of the polymer component. In accord with classical understanding of eutectic solidification, these blends feature a finely phase-separated matrix surrounding primary crystals of the small-molecular species. The combination of large interfacial area and component connectivity yield a desired microstructure for use in bulk-heterojunctions.
Ternary blends for polymer bulk heterojunction solar cells
Polymer International, 2013
The objective of this mini-review is to outline current major ternary blends used in the active layer of polymer bulk heterojunction photovoltaic solar cells and to give an insight into the direction of the field. The use of a third-component material in polymer − fullerene blends is described in two sections. On the one hand, the first family of solid state additives enables us to enlarge photon collection by expanding the action spectra of the solar cells. The second section deals with materials used to engineer bulk heterojunction morphology at the nanoscale. The different approaches explored for many of the ternary blend systems suggest the great potential of such mixtures to significantly improve the optoelectronic properties of solar cells on a long-term basis. Polymer bulk heterojunction solar cells www.soci.org Fabrice Goubard is a full professor of Chemistry in LPPI at the University of Cergy-Pontoise with expertise in hybrid and organic photovoltaic applications. He received his PhD (1995) degree in Solid Chemistry from P. et M. Curie University (Paris, France). He joined LPPI in 1996 as an assistant professor. His research currently deals with pi-conjugated molecular glasses for hybrid photovoltaic devices, and stabilization of organic photovoltaic cells with IPN network.