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Flow-enhanced solution printing of all-polymer solar cells.

Diao Y, Zhou Y, Kurosawa T, Shaw L, Wang C, Park S, Guo Y, Reinspach JA, Gu K, Gu X, Tee BC, Pang C, Yan H, Zhao D, Toney MF, Mannsfeld SC, Bao Z - Nat Commun (2015)

Bottom Line: Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes.The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation.We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA [2] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.

ABSTRACT
Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

No MeSH data available.


Related in: MedlinePlus

FLUENCE for controlling microphase separation of printed all-polymer solar cells.FLUENCE stands for ‘fluid-enhanced crystal engineering'. (a) Schematic of the FLUENCE method implemented on the solution shearing platform. (b) Schematic of the microphase-separated morphology in bulk heterojunction solar cell and the molecular structures of the electron-donor and electron-acceptor polymers used in this study. (c) Scanning electron microscope images of the microstructured printing blade, scale bar 2 μm (top), 5 μm (bottom). The white dotted line indicates the size of the simulation box in the xy plane. (d) Finite element simulation results (stream-line representation) of the flow field between the microstructured printing blade and the substrate. The simulated printing speed is 50 μm s−1. The colour scale of the fluid velocity is shown to the right. In this case, the flow is mainly driven by solvent evaporation instead of the printing motion. The cut plane shown (middle image) lies parallel to the substrate, approximately equidistant to the blade and the substrate in the z direction. The hypothesized polymer conformation change, alignment and aggregation/crystallization under extensional and shear flow are depicted in the simulated flow field (right image).
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f1: FLUENCE for controlling microphase separation of printed all-polymer solar cells.FLUENCE stands for ‘fluid-enhanced crystal engineering'. (a) Schematic of the FLUENCE method implemented on the solution shearing platform. (b) Schematic of the microphase-separated morphology in bulk heterojunction solar cell and the molecular structures of the electron-donor and electron-acceptor polymers used in this study. (c) Scanning electron microscope images of the microstructured printing blade, scale bar 2 μm (top), 5 μm (bottom). The white dotted line indicates the size of the simulation box in the xy plane. (d) Finite element simulation results (stream-line representation) of the flow field between the microstructured printing blade and the substrate. The simulated printing speed is 50 μm s−1. The colour scale of the fluid velocity is shown to the right. In this case, the flow is mainly driven by solvent evaporation instead of the printing motion. The cut plane shown (middle image) lies parallel to the substrate, approximately equidistant to the blade and the substrate in the z direction. The hypothesized polymer conformation change, alignment and aggregation/crystallization under extensional and shear flow are depicted in the simulated flow field (right image).

Mentions: Herein we describe the design concept of FLUENCE for controlling polymer crystallization and therefore microphase separation in BHJ solar cell systems. We previously demonstrated the use of FLUENCE for large-area coating of aligned single-crystalline arrays of small molecule organic transistors38. In this work, however, the flow design is based on an entirely different concept (discussed below) given that polymer crystallization is strongly influenced by chain conformation dynamics, distinct from small molecules. The effect of fluid flow on polymer phase behaviour has been studied extensively in the field of polymer rheology, in particular for bulk commodity polymers such as polypropylene424344 and recently, biomolecules such as DNA45. However, these concepts have not been explored for the solution printing of solar cell materials. Flow-induced nucleation has been observed in dilute polymer solutions at concentrations (∼2 wt%) and shear rates (4–40 s−1) comparable with those of our processing conditions4647. This phenomenon is closely related to flow-induced changes in polymer conformations. In particular, flow-induced chain extension and alignment are deemed responsible for expedited polymer crystallization due to a lowered entropic barrier to the formation of ordered structures44. Among the various flow types, extensional flow has been shown to be the most effective in inducing crystallization by means of stretching the polymer chains424344; shear flow was also found to promote crystallization kinetics, although much less effectively, by possibly increasing chain alignment444849. In meniscus-guided solution coating methods40, such as the solution shearing method36373850 used in this study, shear flow is the dominant flow type with minimal extensional flow characteristics. To induce extensional flow as well as to increase the shear rate across various printing conditions, we pattern the printing blade with micropillar arrays, which ‘comb' the ink during the printing process to direct the microphase separation between the polymeric electron donor and acceptor materials (Fig. 1a). Finite element-based fluid simulation results show that the presence of micropillars effectively induced extensional flow and enhanced the shear rate (Fig. 1d). Using the pillar arrays shown in Fig. 1c, the maximum extensional strain rate (δv/δx) increased by ∼2 orders of magnitude to ∼500 s−1, and the maximum shear rate (δv/δy) increased by ∼40 times to over 1,000 s−1 as compared with the case of the unstructured blade. These enhancements are attributed to several key design parameters deduced from fluid simulations. First, small pillar spacing along y axis (perpendicular to the shearing direction) is critical to expediting the flow in between the pillars and for inducing high shear rates. Second, the staggered arrangement of the pillar array as well as the close row spacing along x axis are important for generating a high extensional strain rate in the direction of the flow (Fig. 1d). We hypothesize that the high extensional strain rate facilitates stretching of the polymer chains, which are subsequently aligned under high shear rate (Fig. 1d). Both effects cooperate to promote polymer nucleation and drive microphase separation10 between the donor and acceptor phase (Fig. 1b). To verify our design concept, we later show that increasing the pillar gap and the row spacing by over tenfold diminishes the effect of FLUENCE on film morphology (see Discussion). It is worth noting that evidence of flow-induced crystallization have been presented and studied in depth in the context of isotatic polypropylene crystallization from melt44515253. Lamberti et al. have shown that flow-induced crystallization is due not only to extensional flow, but also to shear flow-induced orientation ordering, which has been observed in melt. The flow-induced orientation decreased the entropy of phase change shown using computational approach. Their studies further support our design concept and hypothesis.


Flow-enhanced solution printing of all-polymer solar cells.

Diao Y, Zhou Y, Kurosawa T, Shaw L, Wang C, Park S, Guo Y, Reinspach JA, Gu K, Gu X, Tee BC, Pang C, Yan H, Zhao D, Toney MF, Mannsfeld SC, Bao Z - Nat Commun (2015)

FLUENCE for controlling microphase separation of printed all-polymer solar cells.FLUENCE stands for ‘fluid-enhanced crystal engineering'. (a) Schematic of the FLUENCE method implemented on the solution shearing platform. (b) Schematic of the microphase-separated morphology in bulk heterojunction solar cell and the molecular structures of the electron-donor and electron-acceptor polymers used in this study. (c) Scanning electron microscope images of the microstructured printing blade, scale bar 2 μm (top), 5 μm (bottom). The white dotted line indicates the size of the simulation box in the xy plane. (d) Finite element simulation results (stream-line representation) of the flow field between the microstructured printing blade and the substrate. The simulated printing speed is 50 μm s−1. The colour scale of the fluid velocity is shown to the right. In this case, the flow is mainly driven by solvent evaporation instead of the printing motion. The cut plane shown (middle image) lies parallel to the substrate, approximately equidistant to the blade and the substrate in the z direction. The hypothesized polymer conformation change, alignment and aggregation/crystallization under extensional and shear flow are depicted in the simulated flow field (right image).
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Related In: Results  -  Collection

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f1: FLUENCE for controlling microphase separation of printed all-polymer solar cells.FLUENCE stands for ‘fluid-enhanced crystal engineering'. (a) Schematic of the FLUENCE method implemented on the solution shearing platform. (b) Schematic of the microphase-separated morphology in bulk heterojunction solar cell and the molecular structures of the electron-donor and electron-acceptor polymers used in this study. (c) Scanning electron microscope images of the microstructured printing blade, scale bar 2 μm (top), 5 μm (bottom). The white dotted line indicates the size of the simulation box in the xy plane. (d) Finite element simulation results (stream-line representation) of the flow field between the microstructured printing blade and the substrate. The simulated printing speed is 50 μm s−1. The colour scale of the fluid velocity is shown to the right. In this case, the flow is mainly driven by solvent evaporation instead of the printing motion. The cut plane shown (middle image) lies parallel to the substrate, approximately equidistant to the blade and the substrate in the z direction. The hypothesized polymer conformation change, alignment and aggregation/crystallization under extensional and shear flow are depicted in the simulated flow field (right image).
Mentions: Herein we describe the design concept of FLUENCE for controlling polymer crystallization and therefore microphase separation in BHJ solar cell systems. We previously demonstrated the use of FLUENCE for large-area coating of aligned single-crystalline arrays of small molecule organic transistors38. In this work, however, the flow design is based on an entirely different concept (discussed below) given that polymer crystallization is strongly influenced by chain conformation dynamics, distinct from small molecules. The effect of fluid flow on polymer phase behaviour has been studied extensively in the field of polymer rheology, in particular for bulk commodity polymers such as polypropylene424344 and recently, biomolecules such as DNA45. However, these concepts have not been explored for the solution printing of solar cell materials. Flow-induced nucleation has been observed in dilute polymer solutions at concentrations (∼2 wt%) and shear rates (4–40 s−1) comparable with those of our processing conditions4647. This phenomenon is closely related to flow-induced changes in polymer conformations. In particular, flow-induced chain extension and alignment are deemed responsible for expedited polymer crystallization due to a lowered entropic barrier to the formation of ordered structures44. Among the various flow types, extensional flow has been shown to be the most effective in inducing crystallization by means of stretching the polymer chains424344; shear flow was also found to promote crystallization kinetics, although much less effectively, by possibly increasing chain alignment444849. In meniscus-guided solution coating methods40, such as the solution shearing method36373850 used in this study, shear flow is the dominant flow type with minimal extensional flow characteristics. To induce extensional flow as well as to increase the shear rate across various printing conditions, we pattern the printing blade with micropillar arrays, which ‘comb' the ink during the printing process to direct the microphase separation between the polymeric electron donor and acceptor materials (Fig. 1a). Finite element-based fluid simulation results show that the presence of micropillars effectively induced extensional flow and enhanced the shear rate (Fig. 1d). Using the pillar arrays shown in Fig. 1c, the maximum extensional strain rate (δv/δx) increased by ∼2 orders of magnitude to ∼500 s−1, and the maximum shear rate (δv/δy) increased by ∼40 times to over 1,000 s−1 as compared with the case of the unstructured blade. These enhancements are attributed to several key design parameters deduced from fluid simulations. First, small pillar spacing along y axis (perpendicular to the shearing direction) is critical to expediting the flow in between the pillars and for inducing high shear rates. Second, the staggered arrangement of the pillar array as well as the close row spacing along x axis are important for generating a high extensional strain rate in the direction of the flow (Fig. 1d). We hypothesize that the high extensional strain rate facilitates stretching of the polymer chains, which are subsequently aligned under high shear rate (Fig. 1d). Both effects cooperate to promote polymer nucleation and drive microphase separation10 between the donor and acceptor phase (Fig. 1b). To verify our design concept, we later show that increasing the pillar gap and the row spacing by over tenfold diminishes the effect of FLUENCE on film morphology (see Discussion). It is worth noting that evidence of flow-induced crystallization have been presented and studied in depth in the context of isotatic polypropylene crystallization from melt44515253. Lamberti et al. have shown that flow-induced crystallization is due not only to extensional flow, but also to shear flow-induced orientation ordering, which has been observed in melt. The flow-induced orientation decreased the entropy of phase change shown using computational approach. Their studies further support our design concept and hypothesis.

Bottom Line: Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes.The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation.We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA [2] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.

ABSTRACT
Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

No MeSH data available.


Related in: MedlinePlus