Effect of 2-D Delocalization on Charge Transport and Recombination in Bulk-Heterojunction Solar Cells

Österbacka, Ronald; Pivrikas, Almantas; Juška, G.; Poškus, Andrius; Aarnio, Harri; Sliaužys, Gytis; Genevičius, K.; Arlauskas, K.; Sariciftci, Niyazi Serdar

doi:10.1109/jstqe.2010.2048746

Cited by 17 publications

(20 citation statements)

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“…Integration of Equation ( 1) yields that the decay dynamics should be proportional to t − 1 at long times. G. Juška et al [ 9,10 ] have suggested that the charge transport in lamellar systems are mainly restricted to two dimensions, since the mobility in the lamellar plane is signifi cantly larger than the out-of-plane mobility. This fact suggests that in the case of crystalline blends the standard 3D Langevin formalism should be replaced by a 2D one (2D Langevin).…”

Section: Wileyonlinelibrarycommentioning

confidence: 99%

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

Nyman

Sandberg

Österbacka

2014

Advanced Energy Materials

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developed a theory to describe the rate at which anions and cations recombine in an ion gas; the rate depends only on the velocity at which the ions move in their mutual Coulomb fi eld since it is assumed that once the ions are close enough to each other recombination (ion bond formation) is instantaneous. Langevin theory does not describe the reverse process (anioncation formation); the time reversed process was described by Onsager and further extended by Braun. [ 2,3 ] When applying standard Langevin theory to BHJ blends two basic assumptions are made, namely the charge transport in the active material is isotropic and the rate of nongeminate polaron pair formation is much lower than the rate of polaron pair recombination. If these assumptions hold the recombination rate is simply given by the probability of an electron and a hole to meet in coordinate space, so that R = β L np , and the Langevin recombination coeffi cient β L is:where e = elementary charge, µ n(p) = the electron (hole) mobility, ε = relative permittivity, and ε 0 = the vacuum permittivity. In general either one of these assumptions-or both, may not be valid. For example, if the charge-carrier mobility is highly anisotropic an electron will not necessarily recombine with the hole that is closest; it will recombine with the one that is most easily accessible. This could occur for example in lamellar systems; it has been shown that the mobility in the plane of a polymer lamella can be up to 100 times the out of plane mobility. [ 4 ] Several BHJ blends have been reported to show a bimolecular recombination that is signifi cantly reduced as compared with Langevin, [5][6][7] in particular a reduction in the recombination rate of a factor of 10 2 -10 4 has been reported in the BHJ blend poly(3-hexylthiophene) (P3HT) and PCBM. [ 5 ] In these cases a recombination prefactor, defi ned as / L ξ β β = , is typically introduced. Several models have been proposed to clarify the underlying mechanism of reduced recombination, some of which can be directly related to restricted validity of the assumptions in Langevin theory listed above. Koster et al. [ 8 ] noted that the complex created when an electron and a hole meet in coordinate space (charge encounter complex) can be resplit into a free electron and hole with a certain probability. Murthy et al. [ 7 ] noted that for several BHJ systems exhibiting reduced

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Section: Wileyonlinelibrarycommentioning

confidence: 99%

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

Nyman

Sandberg

Österbacka

2014

Advanced Energy Materials

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“…and kT = 0.025 eV were used for the calculations; the same values were used by Deibel et al 16 and Rubel et al 28 Figure 3 shows that a dissociation efficiency close to unity, as found experimentally, 32 can be obtained for realistic effective masses m eff ≥ m e only at high electric fields F > 10 7 V/m. However, experimental studies 33,34 show that the electric field is not a determining parameter of the photogeneration flux in solar cells.…”

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Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

Baranovskiǐ

Wiemer

Nenashev

et al. 2012

J. Phys. Chem. Lett.

101

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The decisive feature of any material designed for photovoltaic applications is the dissociation efficiency of photogenerated excitons. This efficiency is essentially governed by the Coulomb attraction between electrons and holes. Because the dielectric constant in organic materials is rather low (ε r ≈ 3), the exciton binding energy is much larger than the thermal energy at room temperature, so that thermally governed dissociation seems improbable. Experiments show that while the dissociation probability for electron−hole pairs is indeed very low in the bulk samples, it becomes close to unity at intrinsic interfaces between two organic materials, an electron donor (usually a conjugated polymer) and an electron acceptor (usually a fullerene derivative). The driving force for this dissociation is still a matter of controversy. This Perspective provides a theoretical analysis of possible mechanisms for the efficient dissociation of electron−hole pairs at internal organic interfaces despite the strong Coulomb attraction between the charges. I t is one of the main challenges in the research on organic materials to reveal the mechanism responsible for the efficient dissociation of electron−hole pairs (EHPs) created by light. The interest of researchers in the dissociation problem is caused mainly by perspectives of photovoltaic applications of organic semiconductors. Such materials usually exhibit very high light absorption coefficients, which, combined with lowcost processing, makes them promising for applications in solar cells. 1,2 However, it is not the efficient light absorption itself but rather the combination of the efficient absorption with the efficient photocurrent generation that makes a material favorable for photovoltaic applications. The decisive step in photocurrent generation is the dissociation of EHPs created by light. In the dissociation process, the electron and hole must overcome their mutual Coulomb attraction, determined by the energywhere e is the elementary charge, ε r is the relative dielectric constant of the surrounding media, ε 0 is the permittivity of vacuum, and r is the electron−hole separation. While in inorganic semiconductors with ε r > 10 the binding energy of excitons is on the order of 10 meV, in organic semiconductors, in which ε r is typically between 2 and 4, 3 the binding energy of excitons is on the order of 1 eV. 4 The puzzling question arises then regarding the mechanism that could provide an efficient dissociation of EHPs with such a huge binding energy as compared to the thermal energy at room temperature, kT ≃ 0.025 eV.Numerous experimental and theoretical studies have been dedicated to the dissociation problem of EHPs in organic semiconductors. There is no chance to review all of them in the current report. Interested readers can find a comprehensive description of the research field in recent review articles. 1,3 Here, we only briefly describe the state of the research field.The first organic solar cells tested in the 1970s used a single material, sandwiched...

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“…In this system, both Langevin and non-Langevin carrier recombination have been observed, depending on the efficiency of the solar cell. [11][12][13][14] It is important to note that Langevin-type recombination is expected in low-mobility materials, such as polymers or disordered organic semiconductors.…”

Section: Resultsmentioning

confidence: 99%

“…Further description of the CELIV experimental setup and film preparation can be found elsewhere. 12 In Fig. 4, CELIV current transients [ Fig.…”

Section: Resultsmentioning

confidence: 99%

Extraction of photogenerated charge carriers by linearly increasing voltage in the case of Langevin recombination

et al. 2011

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Charge extraction by linearly increasing voltage (CELIV) is a powerful and widely used technique for studying charge transport physics, particularly in disordered systems such as organic semiconductors. In this article, we show that CELIV photocurrent transients are strongly dependent on experimental conditions, such as the light intensity and absorption profile. With this in mind, we introduce a universal correction factor that qualitatively extends previously derived CELIV equations, allowing carrier mobility to be estimated at various photogenerated carrier concentrations and, most importantly, photogeneration profiles. In addition, we demonstrate how the CELIV technique can be conveniently used to determine precisely the presence of Langevin bimolecular carrier recombination.

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Effect of 2-D Delocalization on Charge Transport and Recombination in Bulk-Heterojunction Solar Cells

Cited by 17 publications

References 50 publications

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

Extraction of photogenerated charge carriers by linearly increasing voltage in the case of Langevin recombination

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