Device Characteristics of Bulk-Heterojunction Polymer Solar Cells are Independent of Interfacial Segregation of Active Layers

Wang, He; Gomez, Enrique D.; Kim, Jong-Bok; Guan, Zelei; Jaye, Cherno; Fischer, Daniel A.; Kahn, Antoine; Loo, Yueh Lin

doi:10.1021/cm200320u

Cited by 71 publications

(109 citation statements)

References 21 publications

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“…The enrichment layer is followed by a small depletion zone. Recent XPS, 15 19,50 and DSIMS experiments also indicated the existence of P3HT enrichment layer at the free surface; therefore, the vertical composition profile of P3HT from the present study is in good agreement with experiments. Lower surface energy of P3HT as compared to PCBM is usually mentioned as the reason for the thin enrichment layer formation.…”

Section: Resultssupporting

confidence: 89%

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

Lee

Wodo

Ganapathysubramanian

et al. 2014

ACS Appl. Mater. Interfaces

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The nanomorphologies of the bulk heterojunction (BHJ) layer of polymer solar cells are extremely sensitive to the electrode materials and thermal annealing conditions. In this work, the correlations of electrode materials, thermal annealing sequences, and resultant BHJ nanomorphological details of P3HT:PCBM BHJ polymer solar cell are studied by a series of large-scale, coarse-grained (CG) molecular simulations of system comprised of PEDOT:PSS/ P3HT:PCBM/Al layers. Simulations are performed for various configurations of electrode materials as well as processing temperature. The complex CG molecular data are characterized using a novel extension of our graph-based framework to quantify morphology and establish a link between morphology and processing conditions. Our analysis indicates that vertical phase segregation of P3HT:PCBM blend strongly depends on the electrode material and thermal annealing schedule. A thin P3HT-rich film is formed on the top, regardless of bottom electrode material, when the BHJ layer is exposed to the free surface during thermal annealing. In addition, preferential segregation of P3HT chains and PCBM molecules toward PEDOT:PSS and Al electrodes, respectively, is observed. Detailed morphology analysis indicated that, surprisingly, vertical phase segregation does not affect the connectivity of donor/acceptor domains with respective electrodes. However, the formation of P3HT/PCBM depletion zones next to the P3HT/PCBM-rich zones can be a potential bottleneck for electron/hole transport due to increase in transport pathway length. Analysis in terms of fraction of intraand interchain charge transports revealed that processing schedule affects the average vertical orientation of polymer chains, which may be crucial for enhanced charge transport, nongeminate recombination, and charge collection. The present study establishes a more detailed link between processing and morphology by combining multiscale molecular simulation framework with an extensive morphology feature analysis, providing a quantitative means for process optimization.

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Section: Resultssupporting

confidence: 89%

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

Lee

Wodo

Ganapathysubramanian

et al. 2014

ACS Appl. Mater. Interfaces

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“…Effectively, the entire volume of device contributes to charge transport, regardless of processing conditions. Further, this high bulk connectivity of all three phases explains the observation by Wang et al 55 regarding contact area with electrode. They speculated that as long as contact area between fullerene and cathode is nonzero (even as low as 3%), it enables electron extraction.…”

Section: Connectivity Of Domains To Relevant Electrodes and Tortuositsupporting

confidence: 58%

Quantifying organic solar cell morphology: a computational study of three-dimensional maps

Wodo

Roehling

Moulé

et al. 2013

Energy Environ. Sci.

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Establishing how fabrication conditions quantitatively affect the morphology of organic blends opens the possibility of rationally designing higher efficiency materials, yet such a relationship remains elusive. One of the major challenges stems from incomplete three-dimensional representations of morphology, which is due to the difficulties of performing accurate morphological measurements. Recently, three-dimensional measurements of mixed organic layers using electron tomography with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) provided maps of morphology with high resolution and detail. Using a simple, yet powerful, computational tool kit, these complex 3D datasets are converted into a set of physically meaningful morphology descriptors. These descriptors provide means for converting these large, complicated data sets (∼5×10 7 voxels) into simple, descriptive parameters, enabling a quantitative comparison between morphologies fabricated under different conditions. A set of P3HT:endohedral fullerene bulk-heterojunctions, fabricated under conditions specifically chosen to yield a wide range of morphologies, are examined. The effects of processing conditions and electrode presence on interfacial area, domain size distribution, connectivity, and tortuosity of charge transport paths are herein determined directly from real-space data for the first time. Through this characterization, quantitative insights into the role of processing on morphology are provided, as well as a more complete picture of the consequences of a three-phase morphology. The analysis demonstrates a methodology which can enable a deeper understanding into morphology control.

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“…Despite the low contrast, the authors of these studies were able to reconstruct the three-dimensional structure from a series of images taken at different tilt angles. Analysis of the 3D morphologies suggested that since less P3HT fibers were found at the top interface, the top surface was PCBM-rich; this is in direct contradiction with multiple NEXAFS and XPS experiments [11][12][13]. We surmise that two difficulties may be responsible for the apparent disagreement.…”

Section: X-ray and Electron Microscopymentioning

confidence: 56%

“…Thus, the small regions of high conduction visible in Fig. 1C could be due to regions of exposed PCBM; NEXAFS studies have determined that about 3% of PCBM exists at the air interface [12].…”

Section: Scanning Probe Microscopymentioning

confidence: 99%

“…AFM, however, is not suitable for all systems. Mixtures based on regioregular poly(3-hexylthiophene) (P3HT)/PCBM, for example, have finer morphologies and consequently do not exhibit the same structure within the film as the surface of the film; this is in part due to P3HT selectively wetting the top surface of P3HT/PCBM films [11][12][13].…”

Section: Scanning Probe Microscopymentioning

confidence: 99%

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Characterization of the mesoscopic structure in the photoactive layer of organic solar cells: A focused review

et al. 2013

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Device Characteristics of Bulk-Heterojunction Polymer Solar Cells are Independent of Interfacial Segregation of Active Layers

Abstract: The ability to reverse the composition profile of active layers by delaminating and transferring P3HT:PCBM thin films has allowed us to probe directly the sole influence of interfacial segregation on solar cell device characteristics on the same device platform.

Cited by 71 publications

References 21 publications

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

Quantifying organic solar cell morphology: a computational study of three-dimensional maps

Characterization of the mesoscopic structure in the photoactive layer of organic solar cells: A focused review

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