Dynamic Stark effect as a probe of the evolution of geminate electron-hole pairs in a conjugated polymer

Gulbinas, Vidmantas; Kananavičius, Robertas; Valkūnas, Leonas; Bäßler, H.

doi:10.1103/physrevb.66.233203

Cited by 28 publications

(26 citation statements)

References 13 publications

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“…General aspects of the EDA technique were discussed in the literature. 18,20 The steady-state EA spectra of pristine Cy3-P, pristine C 60 and C 60 /Cy3-P (30/20) bilayer devices measured under an applied reverse bias are presented in Figure 2. Here and further in the text the label C 60 /Cy3-P (−/−) denotes the thickness of the corresponding layers of the device.…”

Section: Resultsmentioning

confidence: 99%

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study

Jonghe-Risse²,

et al. 2015

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Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye/fullerene organic solar cells enables one to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron−hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers' generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

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Section: Resultsmentioning

confidence: 99%

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study

Jonghe-Risse²,

et al. 2015

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“…We use the timeresolved Electric Field-Induced Second Harmonic (EFISH) method 27 , in a pump-probe configuration to probe the electric field dynamics with subpicosecond time resolution. This technique is similar to probing the electric field dynamics by Stark effect 28,29 , but the analysis is more straightforward. The dynamics of the electric field allows us to monitor the polarization of molecular charge densities and drift of charge carriers.…”

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Visualizing charge separation in bulk heterojunction organic solar cells

et al. 2013

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Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

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“…Following photogeneration in a PCBM diode under reverse bias, singlets are separated into electron-hole pairs. These charge pairs act as dipoles leading to a photoinduced polarization opposed to the applied field [17]. As they drift farther apart their associated dipole moment increases and the electric field inside the illuminated area is gradually screened as reflected by a 75% reduction of the Stark feature.…”

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Photoinduced Transient Stark Spectroscopy in Organic Semiconductors: A Method for Charge Mobility Determination in the Picosecond Regime

et al. 2006

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Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium. DOI: 10.1103/PhysRevLett.96.106601 PACS numbers: 72.80.Rj, 72.80.Ng, 78.47.+p, 78.66.Tr Charge carrier mobility in organic semiconductors is a critical material property that determines the performance characteristics of many electronic devices, including fieldeffect transistors [1], light-emitting diodes [2], and solar cells [3]. For this reason, many experimental techniques and theoretical modeling have been devoted to its measure and interpretation [4 -7]. It turns out that different experiments provide different values of carrier mobility in the same material, due to varying experimental conditions and different device architectures. For instance, it is found that the hole mobility of common polymer semiconductors is several orders of magnitude lower when tested in lightemitting diodes than in field-effect transistors [8,9]. Such variations make material evaluation difficult if not impossible.In amorphous organic materials charge carrier mobility is often the average of a number of slowing down processes where defects and traps dominate. Ideally one would like to measure the carrier mobility in a pure, defect free material, so meaningful comparison between different experiments and theory would be allowed. Close to this ideal case is the use of time resolved microwave conductivity technique, an electrodeless method that monitors charge dipoles absorption in the transient regime [7,10]. The time resolution of this technique is limited, however, to the nanosecond domain. Here we present an alternative approach, based on photoinduced transient Stark effect in which optical probing is exploited for extracting mobility values in the picosecond time regime. A related technique, exploiting the Franz-Keldysh effect in extended band states, was successfully employed by Shank et al. to measure nonequilibrium carrier transport in inorganic semiconductors [11][12][13]. The validity of our method relies on the fact that charge transport is monitored in situ on the compound without current extraction, avoiding pitfalls caused by carrier migration along macroscopic regions of the sample as well as by carrier extraction at semiconductor-metal electrode interfaces. Mobilities obtained in this way can be considered as upper limit values, unaffected by limiting processes such as trapping or recombination. This information is intrinsic and constitutes a direct feedback for the design of new materials with improved...

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Dynamic Stark effect as a probe of the evolution of geminate electron-hole pairs in a conjugated polymer

Cited by 28 publications

References 13 publications

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study

Visualizing charge separation in bulk heterojunction organic solar cells

Photoinduced Transient Stark Spectroscopy in Organic Semiconductors: A Method for Charge Mobility Determination in the Picosecond Regime

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