Control of Electric Field Strength and Orientation at the Donor–Acceptor Interface in Organic Solar Cells

Liu, Albert; Zhao, Shanbin; Rim, Seung-Bum; Wu, Jingshi; Könemann, Martin; Erk, Peter; Peumans, Peter

doi:10.1002/adma.200702554

Cited by 91 publications

(99 citation statements)

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“…Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .…”

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Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Dibb

Muth

Kirchartz

et al. 2013

Sci Rep

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While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...

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confidence: 99%

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Dibb

Muth

Kirchartz

et al. 2013

Sci Rep

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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%

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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“…To explain this mystery, various researchers have been actively discussing the exciton dissociation process and have proposed several explanatory mechanisms such as the contribution of entropy (dimensional effect), the hot charge-transfer (CT) state, doping or defects, and structural heterogeneity. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Among these, a strong candidate is the hot CT mechanism. 1, [22][23][24][25][26][27][28] In our previous study, we proposed a theoretical method to consider the hot CT state and examined the exciton dissociation process at the donor-acceptor interface.…”

Section: Introductionmentioning

confidence: 99%

Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

Shimazaki

Nakajima

2016

The Journal of Chemical Physics

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This paper discusses the exciton dissociation process at the donor-acceptor interface in organic photocells. In our previous study, we introduced a local temperature to handle the hot chargetransfer (CT) state and calculated the exciton dissociation probability based on the 1D organic semiconductor model [T. Shimazaki and T. Nakajima, Phys. Chem. Chem. Phys. 17, 12538 (2015)]. Although the hot CT state plays an essential role in exciton dissociations, the probabilities calculated are not high enough to efficiently separate bound electron-hole pairs. This paper focuses on the dimensional (entropy) effect together with the hot CT state effect and shows that cooperative behavior between both effects can improve the exciton dissociation process. In addition, we discuss cooperative effects with site-disorders and external-electric-fields. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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Control of Electric Field Strength and Orientation at the Donor–Acceptor Interface in Organic Solar Cells

Abstract: Electrical doping is essential to achieve efficient photocurrent extraction in small‐molecular‐weight organic solar cells. It is shown that such doping creates strong electric fields at the donor‐acceptor interface that prevent geminate electron–hole recombination.

Cited by 91 publications

References 27 publications

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

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

Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

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