Excited State and Charge Photogeneration Dynamics in Conjugated Polymers

Scheblykin, Ivan G.; Yartsev, Arkady; Pullerits, Tõnu; Gulbinas, Vidmantas; Sundström, Villy

doi:10.1021/jp068864f

Cited by 237 publications

(314 citation statements)

References 174 publications

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“…It is gratifying to note that the yield we report, η = 8 ± 2 %, is comparable to values obtained by means of complementary techniques 38,39 . This last comparison supports the reasoning that donor and acceptor moieties within the polymer backbone are promoting charge separation and polaron pair formation, whereas in homopolymers, such as P3HT, charge separation is less probable and eventually driven by the energetic disorder between the polymer chromophores 40 . …”

Section: Lowsupporting

confidence: 75%

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

et al. 2012

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Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donoracceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η = 8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.

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Section: Lowsupporting

confidence: 75%

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

et al. 2012

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“…[43,48,52,53,[93][94][95] A delocalization of the primary photoexcitation over at least 11 nm has been reported for MEH-PPV. [48] Although the primary photoexcitation in conjugated polymers is often referred to as an exciton [33] or hot exciton, [96] we do not believe that the binding of the opposite charges in this non-thermalized and highly delocalized initial state is strong enough to formally call it an exciton. Instead, we use the semiconductor formalism and describe it as a correlated, but not bound, pair of a mobile electron and hole, formed by a π-π* interband transition.…”

Section: Photoexcitation and Charge Transfermentioning

confidence: 93%

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Cowan

Banerji

Leong

et al. 2012

Adv Funct Materials

305

252

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This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

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“…2,39,41 On the other hand, evidence exists for a more molecular approach, where the primary photoexcitation is already a bound exciton. [42][43][44] In the present article, we will more neutrally use "excited state" as the terminology and show that its nature depends on both disorder and on the time scale at which it is observed.…”

Section: Classic Conjugated Homopolymersmentioning

confidence: 99%

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

2013

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In this feature article, we review and examine evidence that the primary photoexcited species in conjugated polymers is considerably delocalized. Localization occurs via a series of complex relaxation mechanisms on the <200 femtosecond time scale. We show that short-lived delocalization in the neutral excited state and charge separated state of bulk heterojunction blends might play an essential role in ensuring efficient formation of free charge carriers for photovoltaic applications. Finally, the additional parameter of intramolecular charge transfer character in the excited state of more recently developed donor-acceptor copolymers is discussed. Both delocalization and partial charge transfer reduce the binding of the electron and hole in photoexcited organic semiconductors and can help to overcome the bottleneck to macroscopic current generation in polymer solar cells.

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Excited State and Charge Photogeneration Dynamics in Conjugated Polymers

Cited by 237 publications

References 174 publications

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

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