Why Holes and Electrons Separate So Well in Polymer/Fullerene Photovoltaic Cells

McMahon, David P.; Cheung, David L.; Troisi, Alessandro

doi:10.1021/jz201325g

Cited by 183 publications

(223 citation statements)

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“…Design rules targeting enhanced exciton and charge diffusion or exciton splitting are, however, hard to come by, since the underlying microscopic mechanisms are not well understood. Efficient exciton dissociation, for example, has been attributed to the assistance of charge separation by a gradient in the free-energy landscape [132,133], structural heterogeneity as a function of distance to the interface [134], doping and charged defects [135], increase in entropy as the electron and hole move away from the interface [136], formation of hot CT states [137], or long-range tunneling [138]. Tuning optical absorption profiles, by contrast, is a more manageable approach to enhance the external quantum efficiency of single-junction devices.…”

Section: The Acceptor-donor-acceptor Puzzlementioning

confidence: 99%

“…Pathways for charge separation as a microscopically poorly understood process [134,136,138,161,[186][187][188][189] are therefore still actively investigated: As a key result of this chapter, we will show how the energy landscape that emerges from mesoscale order provides pushout forces that can drive the charge separation process -in line with the apparent absence of a Coulomb barrier claimed for some systems [187]. In deriving the functional difference with a small photovoltaic gap Γ versus the (b) face-on orientation with large Γ. Gas-phase ionization energies and electron affinities are denoted as IE 0 and EA 0 , respectively.…”

Section: Pathway For Charge Splitting and Detrappingmentioning

confidence: 99%

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Poelking

2018

Springer Theses

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The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors The rational design of organic semiconductors for optoelectronic devices relies on a detailed understanding of how their molecular and morphological structure condition the energetics and dynamics of charged and excitonic states. Investigating the role of molecular architecture, conformation, orientation and packing, this work reveals mechanisms that shape the spatially resolved densities of states in organic, small-molecular and polymeric heterostructures and mesophases. The underlying computational framework combines multiscale simulations of the material morphology at atomistic and coarse-grained resolution with a long-range-polarized embedding technique to resolve the electronic structure of the molecular solid. We show that longrange electrostatic interactions tie the energetics of microscopic states to the mesoscopic structure, with a qualitative and quantitative impact on charge-carrier level profiles across organic interfaces. The computational approach provides quantitative access to the charge-density-dependent opencircuit voltage of planar heterojunctions. The derived and experimentally verified relationships between molecular orientation, architecture, level profiles and open-circuit voltage rationalize the acceptor-donor-acceptor pattern for donor materials in high-performing solar cells. Proposing a pathway for barrier-less dissociation of charge transfer states, we highlight how mesoscale fields generate charge splitting and detrapping forces in systems with finite interface roughness. The associated design rules reflect the dominant role played by lowest-energy configurations at the interface. Acknowledgements 121 Die (nicht-)lokale Zustandsdichte elektronischer Anregungen in organischen Halbleitern List of Publications 123Bibliography 125 Chapter 1 Organic Electronics in a NutshellThe discovery of conductive polymers in the 1970s [1,2] -rewarded with the Nobel prize in chemistry in the year 2000 -and subsequent development of the first polymeric electronic devices in the 1980s [3-5] have marked the beginnings of a vast interdisciplinary research field referred to as organic electronics. Drawing from both polymeric and small-molecular semiconductors, organic thin-film transistors, solar cells, light-emitting diodes, photodetectors and sensors name just a few out of many applications that make use of the supreme chemical versatility and mechanical flexibility of the underlying "soft" molecular materials. Furthermore enabling low-temperature solution-based processing, printing and spray coating, organic electronics is set to complement the conventional silicon-based electronics in diverse waysadding functionality and improving sustainability. This introductory chapter will provide a short review of the materials and device physics, as well as of new trends and challenges that emerged in the field over the past decade. MaterialsOrganic semiconductors are molecular materials that exist at the interface between orga...

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Section: The Acceptor-donor-acceptor Puzzlementioning

confidence: 99%

Section: Pathway For Charge Splitting and Detrappingmentioning

confidence: 99%

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Poelking

2018

Springer Theses

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“…13,14 A mechanistic understanding of the charge generation process has been pursued for several years in order to explain these apparent discrepancies. Current opinion can be roughly divided between researchers who emphasize the role of energetic and morphological inhomogeneities [15][16][17][18][19][20][21] and those emphasizing kinetic processes. 22- 34 Guo and Inganas provide a concise review of many the leading viewpoints.…”

Section: Introductionmentioning

confidence: 99%

Charge transfer from delocalized excited states in a bulk heterojunction material

2015

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Charge generation in an organic photovoltaic blend was investigated using transient absorption spectroscopy. In films of pure electron donating material, sub-picosecond spectral oscillations were observed and assigned to torsional modes associated with excited state relaxation and localization. These modes are systematically suppressed in the presence of fullerene, indicating that a significant fraction of charge transfer occurs prior to excited state localization.

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“…Troisi and coworkers pointed out that because of the 938 lower band gap of chain segments within the crystallites, these excitons are repelled 939 by the more disordered donor-acceptor interface [179]. The authors therefore 940 proposed that these excitons split via tunneling of the electron through layers of 941 more distorted polymer chains at the interface into higher and partially delocalized 942 states on the PCBM aggregates (see Fig.…”

mentioning

confidence: 99%

“…Exciton split-up can, however, occur via electron tunneling into partially delocalized states in the PCBM aggregates. Reprinted (adapted) with permission from [179]. Copyright 2011 American Chemical Society.…”

mentioning

confidence: 99%

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Moulé

Neher

Turner

2014

P3HT Revisited – From Molecular Scale to Solar Cell Devices

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Each year we are bombarded with B.Sc. and Ph.D. applications from 8 students that want to improve the world. They have learned that their future depends 9 on changing the type of fuel we use and that solar energy is our future. The hope and 10 energy of these young people will transform future energy technologies, but it will 11 not happen quickly. Organic photovoltaic devices are easy to draw AU1, but the mate-12 rials, processing steps, and ways of measuring the properties of the materials are 13 very complicated. It is not trivial to make a systematic measurement that will 14 change the way other research groups think or practice. In approaching this chapter, 15 we thought about what a new researcher would need to know about organic 16 photovoltaic devices and materials in order to have a good start in the subject. [1][2][3][4]. Similar 43 to the gold rush in the 1800s and the oil boom in the 1900s, intellectual property on 44 new technologies is now the boom industry for innovative people to become rich 45 and influential fast. In the twenty-first century, scientists and engineers are the 46 pioneers and our ideas are the prize. One of the most alluring energy technologies of 47 the past decade has been organic photovoltaics (OPV). This technology is alluring 48 because it could potentially reduce the cost of producing photovoltaic 49 (PV) modules and thereby make solar energy cost-competitive with fossil fuels. 50 As can be seen in Fig. 1, the allure of OPV brought thousands of scientists and 51 engineers into this new field, generating an exponential increase in scientific 52 knowledge (as measured by the number of scientific AU4 articles) in this area. The 53 sharp focus on OPV technology has led to an explosion of interest in enabling 54 technologies such as polymer synthesis, polymer physics, microstructural measure-55 ment techniques, multiscale modeling, photophysics, organic electronics, organic-56 inorganic hybrid materials, etc. All of this intense focus into one research area has 57 also created intense competition between research groups. With so many new 58 scientific articles published yearly, it is impossible to read them all, and repeat or 59 redundant articles have become unfortunately and unavoidably common. Even 60 review articles and books about OPV have proliferated, making production of an 61 original perspective difficult and a complete literature review impractical. We 62 apologize in advance if any important work is not cited here. 63Under this backdrop, we have decided to produce an article that is designed to be 64 helpful to students and postdocs who are entering this field. Rather than focusing on 65 the efficiency of devices or the morphology of materials (subjects that are covered 66 very well elsewhere), we instead focus some attention on how to approach OPV 67 research from a more practical (laboratory-based) perspective. Section 1 introduces A.J. Moulé et al. 68OPV devices, modules, and scale-up. Section 2 discusses fabrication of poly-3- Device Characterist...

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Why Holes and Electrons Separate So Well in Polymer/Fullerene Photovoltaic Cells

Cited by 183 publications

References 37 publications

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

Charge transfer from delocalized excited states in a bulk heterojunction material

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

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